Specific connectivity between photoreceptors and horizontal cells in the zebrafish retina

The functional and morphological connectivity between various horizontal cell (HC) types (H1, H2, H3 and H4), and photoreceptors was studied in zebrafish retina. Since HCs are strongly coupled by gap-junctions and feedback from HCs to photoreceptors strongly depends on connexin (Cx) hemichannels we characterized the various HC Cxs (Cx52.6, Cx52.7, Cx52.9 and Cx55.5) in Xenopus oocytes. All Cxs formed hemichannels which were conducting at physiological membrane potentials. The Cx hemichannels differed in kinetic properties and voltage dependence, allowing for specific tuning of the coupling of HCs and the feedback signal from HCs to cones. The morphological connectivity between the HC layers and the cones was determined next. We used zebrafish expressing GFP under the control of Cx promoters. We found that all HCs showed Cx55.5 promoter activity. Cx52.7 promoter activity was exclusively present in H4 cells while Cx52.9 promoter activity occurred only in H1 cells. Cx52.6 promoter activity was present in H4 cells and in the ventral quadrant of the retina also in H1 cells. Finally we determined the spectral sensitivities of the HC layers. Three response types were found. Monophasic responses were generated by HCs that contacted all cones (H1 cells), biphasic responses were generated by HCs that contacted M-, S- and UV-cones (H2 cells), and triphasic responses were generated by HCs that contacted either S- and UV-cones (H3 cells) or rods and UV-cones (H4 cells). Electronmicroscopy confirms that H4 cells innervate cones. This indicates that rod driven HCs process spectral information during photopic and luminance information during scotopic conditions.